The man responsible for a group of 90 school students on Mt Tongariro yesterday said there was eerie sense of calm before it suddenly erupted.

Paul Lowes, a teacher at Tamatea Intermediate in Napier, was leading the youngsters over the Tongariro Crossing when it started spurting smoke and ash, about 1.30pm.

The group was celebrating reaching their destination - the Ketetahi Springs - when one of the kids pointed towards the Te Maari crater, about a kilometre away.

"We turned around and there she was, she was just starting to blow," Mr Lowes said. "It was a bit of an eerie experience. There was a little bit of a rumbling to start with and then a very peaceful calm and you could just hear the ash, the eruption, the clouds going up into the air.

The students were initially excited by the blast, some started taking photos and teacher Lomi Schaumkel filmed the action.

The video footage showed some of the students, and other people further along the track, turning and running away from the ash cloud.

Mr Lowes said the excitement was quickly surpassed by a sense of urgency to get to safety.

"We stopped in a bit of awe of it really, and then realised what was really happening as it was getting bigger. Then it was - righto, it's time to move everyone out of here," Mr Lowes said.

The children made their way down the mountain and back to Mission Bay Christian Camp, where they will spend the rest of the week.

Mr Lowes said he had been in touch with the Department of Conservation earlier in the day and had every reason to believe the mountain was safe.

He said it will be a school trip he, and the students, will never forget.